Virtually all of the bioreactors presently used for the industrial scale oxidation of particulate solids such as gold-bearing sulfide minerals, copper, zinc, nickel sulphides are slurry reactors, i.e. aerated vessels containing ore slurry which are equipped with mechanical mixer. While considered more efficient than chemical processes for the treatment of sulphidic ore concentrates, current day slurry bioreactors have four major disadvantages:
1. In order to keep the particles (e.g. pyrite) in suspension, it is necessary to apply large amount of mechanical energy for mixing mainly because of the high density of pyrite. PA0 2. The interparticle friction is very strong because of the high solids concentration and the intensive mixing. Since microorganisms grow mainly on the surface of the particles (e.g. sulfide crystals), the particle friction in slurry bioreactors causes detachment of microorganisms which significantly limits the reaction rate. This results in long reaction times (e.g. 3 to 5 days for pyrite). PA0 3. Low oxygen transfer rate due to the presents of suspended solids. PA0 4. Liquid and solid retention times are equal.
The above discussion indicates that both substantial increase in the process efficiency and decrease in energy consumption can be achieved if the bioreaction is carried out under low shear stress conditions and without the need to suspend the particles in liquid.
A recently proposed concept of soil immobilization (see Karamanev et al. "Hydrodynamics of Soil Immobilization in the Immobilized soil Bioreactor" AIChE Journal Vol. 43. No.5 May 1997 pages 1163 to 1170; and Karamanev et al.; "Soil Immobilization: New Concept for Biotreatment of Soil Contaminants" Biothechnology and Bioengineering, Vol 57 No. 4 Feb. 20, 1998 pages 471-476) and the teaching of these publications are incorporated herein by reference. This type of reactor is hereinafter referred to throughout the Application as an immobilized solid particle bioreactor or immobilized bed bioreactor the operation of which is based on the entrapment of solid particles into the pores of a highly porous inert matrix, such as a non-woven textile.
Generally the matrix of an immobilized bed bioreactor will have a wide pore size distribution of between several microns and 2 mm. so that as a slurry containing solids within the same size range as that of the pores of the matrix is circulated repeatedly through the matrix, solid particles get entrapped inside the pores. When soil was used as a solid phase, the resulting structure was named immobilized soil. Immobilized soil particles contain surface-immobilized microorganisms (biofilm). In order to supply the microorganisms with substrate, inorganic salts and oxygen, aqueous solution of these compounds is circulated through the immobilized soil structure.
These immobilized bed bioreactors when used for treating for treatment of solid particles require a system of removing the treated solid particles for further processing, The systems for treating liquids as in the immobilized soil reactors of the prior art or do not provide or teach any systems for removal of solids from the bed.
Due to the depletion of rich ores, mining companies are paying more attention to low-grade ores. The present disclosure will be discussed below with specific reference to gold and the gold industry, but it is to be understood that it is believed the invention has wider application outside of the gold industry (ex other base metals such as copper, zinc and nickel).
The gold in the lower grades of ore (e.g. refractory gold-bearing ores) is encapsulated as fine particles (sometimes on molecular level) in the crystal structure of sulfide (typically pyrite with or without arsenopyrite) ore. This makes it impossible to extract refractory gold by cyanidation since cyanide solution cannot penetrate the pyrite/arsenopyrite crystals and dissolve gold particles, even after fine grinding. To effectively extract gold from these ores, an oxidative pretreatment is necessary to break down the sulphide ore. The most popular methods of pretreatment include nitric acid oxidation, roasting, pressure oxidation and biological oxidation by microorganisms.
Roasting is highly energy consuming and produces off-gases containing sulfur dioxide and arsenic trioxide, which require costly treatment. Both pressure oxidation and oxidation by nitric acid require high pressure, high temperature and/or corrosion-resistant materials.
The biological pretreatment of refractory gold ores is based on the ability of some microorganisms such as Thiobacillus ferrooxidans to oxidize and dissolve pyrite and arsenopyrite, thus liberating the entrapped gold particles. Either whole ore or concentrate can be used for biotreatment. Whole ore is usually treated in heaps while concentrate is treated in bioreactors.
The process of microbial oxidation of concentrate is usually carried out in slurry bioreactors with a unit volume of 200 to 1000 m.sup.3 at temperatures between 20 and 55.degree. C. and under atmospheric pressure. This process is considered to be less expensive and more environmentally friendly than other methods. However, there are some disadvantages of this method. The bioprocess rate is low: it takes usually between 3 and 5 days to treat the sulfide particles, compared to several hours in the case of chemical processes. The energy consumption is relatively high. This is especially important, taking into account that the power cost is usually between 40 and 60% of the total operating cost of the slurry biooxidation. Therefore, the increase in the bioreaction rate and the reduction of energy consumption are important.